You are introduced to to repulsion gel a little way into chapter 5. Follow the chapter and after getting to the top of some dodgy stairs you'll come across a room with the year 1958 and the number 28 beside the door. Make a new save here!
Now you can do it here legitimately with a good few tries. But there is a way to avoid the hard work and only have to reload a few times..... Set your portals up to drop the blue repulsion gel onto the cube, then after the glass starts smashing quickly change your portals to cover the cube in water. If you get the timing just right you should have smashed the glass and cleaned the repulsion gel off the cube leaving it on the floor of the glass cage. Now simply pick it up from where it sits. Trophy unlocked